Can anyone tell me or point me to the performance differences of the Kahrs Avanti vs Original flooring? 13mm vs 15mm. Having a tough time comparing the two (or finding detailed reviews), even though I have used and like my previous Kahrs floor for several years.

* Is the support strength of the two comparable? Meaning, will the 13 flex substantially more than the 15? Should I be concerned?

Background: Installed Kahrs Original Builders in gunstock 8 or so years ago. Floated over concrete slab. Recently had a refrigerator leak that ran under the floor, which had to be cut away to find the extent of the damage. Now I am replacing that floor. I have been nothing but pleased with this floor and will replace with another Kahrs. Though the color and style I like this time is the Walnut Motif in the Avanti Canvas line.

* Should I be concerned that Walnut is less hard than the Oak I previously had? Is Walnut a poor choice? What should I look out for with this Walnut floor?

The janka rating for American walnut is 1005. The rating for red oak is 1250, white oak 1350.
Brazilian walnut rates around 3600.
The rating is derived from how many pounds it takes to embed a ,414 inch diameter ball bearing halfway into the wood, on average, not the end grain.
Even though the walnut flooring will dent a little easier than oak it is a beautiful floor. I have it in my living room.
Kitchens are susceptible to denting from dropping all sorts of things.
You can keep it looking almost like new by filing deep dents with a dark colored pre-finished wood patch, by Woodwise.